1. Field of the Invention
The present invention relates to an uninterruptible power supply and more particularly to down-sizing and lightening weight of an uninterruptible power supply used as a backup power supply of computers and communication equipment.
2. Description of the Related Art
As shown by block diagrams in FIGS. 3 and 4, an uninterruptible power supply normally supplies AC power from a utility line 1 to a load 4 such as a computer and a communication equipment directly through a direct transmission line 10 or through a rectifying circuit 2 and an inverter 3 and when the utility line is interrupted, supplies DC power from a battery 6 to the load 4 by converting to AC power by the inverter 3 to prevent a trouble of the load 4 caused by the interruption.
The battery 6 in the uninterruptible power supply in FIG. 3 is arranged so that AC power from the utility line 1 is normally converted to DC power by the rectifying circuit 2 and the battery 6 is fully float-charged by the DC power and so that the DC power in the battery 6 is supplied to the inverter 3 when the utility line is interrupted.
On the other hand, a battery 6 in the uninterruptible power supply in FIG. 4 is arranged so that AC power from the utility line 1 is normally supplied to a charger 7 for fully charging the battery 6 and so that a switch 5 is turned on to supply DC power from the battery 6 to the inverter 3 when the utility line is interrupted.
Although the arrangement of the uninterruptible power supply in FIG. 3 may be simplified since the battery 6 is charged by output of the rectifying circuit 2, the output voltage of the rectifying circuit 2 has to adjust the floating charge voltage of the battery 6, so that its accuracy needs to be improved.
On the other hand, although the arrangement of the uninterruptible power supply in FIG. 4 is more complicated than that in FIG. 3, the output voltage of the rectifying circuit 2 may be of the input condition of the inverter 3, so that only the accuracy of the output voltage of the charger 7 needs to be improved.
By the way, the inverter 3 in the uninterruptible power supply as described above is provided with a dead time for preventing its arm from short-circuiting via its switching elements. The switching elements also have forward voltage drops during their ON time. Accordingly, in order to obtain AC 100 V sine wave voltage from the inverter 3, its DC input voltage has to be at least 170 V and the final discharge voltage of the battery 6 also has to be more than 170 V. This means that when a lead-acid battery is used for the battery 6, it needs more than 100 cells.
In order to deal with that, it is tried that a boost-chopper circuit is inserted between the rectifying circuit 2 and the inverter 3, for lowering the output voltage of the rectifying circuit 2, increasing the DC input voltage to the inverter 3 and reducing the number of cells in the battery 6.
Accordingly, such uninterruptible power supply as described above had a disadvantage that the cost of the battery 6 accounts for a large amount in the total cost of the supply due to the large number of cells in the battery 6.
Moreover, since many cells are connected in series in the battery 6, there has been a problem in its reliability and man-day.
Furthermore, although the number of cells in the battery 6 may be reduced by inserting the boost-chopper circuit, such another problems caused by the boost-chopper circuit which is operated in high frequency that the efficiency is lowered, the number of parts is increased, the cost is increased and the reliability is decreased have brought about.